Pumping unit with reversing method



Dec. 19, 1961 J. L. WELCHER 3,013,538

PUMPING UNIT WITH REVERSING METHOD Filed Aug. 8, 1958 5 Sheets-Sh t John L. We/cher T ZNVENTOR.

M @6 BY 5%} Dec. 19, 1961 J. L. WELCHER PUMPING UNIT WITH REVERSING METHOD 3 Sheets-Sheet 2 Filed Aug. 8, 1958 INVENTOR.

Dec. 19, 1961 J. L. WELCHER PUMPING UNIT WITH REVERSING METHOD 5 Sheets-Shae};

Filed Aug. 8, 1958 0 s 6 w 8 m 2 p I m n e N A h E m k V 9 8 m W H h W. m 6 11%| L M W 4 m m 9m J J m 4 m u a a L m w/Y m n 4 u a m m n m u luluunnlil. iliiii. 2 FL m 3 "T m m m m m m III. PIIIIMIIIIIL IIIIIIIIIIIIIL n m m m w E 0 ln w 5 ms? n .0. i M M United States Patent Oflice 3,013,538 Patented Dec. 19, 1961 3,013,538 PUMPING UNIT WITH REVERSING METHOD John L. Welcher, Buena Park, Calif., assignor to M-W Hydraulics, Inc, Bakersfield, Calif., a corporation of Caiifornia Filed Aug. 8, 1958, Ser. No. 753,946

6 (Ilaims. (Cl. 121-150) This invention relates in general to fluid actuated motors, and more particularly to a fluid actuated motor which is driven by a constantly running pump pumping a constant volume of fluid, the fluid passages extending etween the pump and the motor including automatically operating valves whereby the fluid motor reciprocates.

This invention relates primarily to a fluid actuated motor assembly which is primarily intended for use as a prime mover for deep well pumps. Such pumps are normally used in the production of oil from an oil well and are the reciprocating type. The pump is normally disposed within the tubing of the oil well within the production zone of such oil well. The pump is connected to the surface by means of a string of sucker rods with the reciprocation of the pump being effected by the sucker rods. Normally, the reciprocation of the sucker rods is accomplished by what is commonly referred to in the industry as a walking beam. However, the walking beam imposes great stresses on the sucker rods inasmuch as it is constantly accelerating, either positively or negatively. The undue stress on the sucker rods often results in the breakage of the sucker rods and as a result the pump and the lower portion of the string of the sucker rod must be fished from the oil well. This is both an expensive and a time consuming operation.

It is recognized that it is old and well known to provide reciprocating fluid motors. Such fluid motors may be reciprocated by providing a valve mechanism which will control the flow of fluid into opposite ends of a cylinder. However, such reciprocating fluid motors also are constantly accelerating and having many of the disadvantages of the walking beam type drive. Furthermore, because of the fact that the weight of the drill string is at all times on the fluid motor, the load imposed upon the fluid motor is different from that normally occurring and as a result the normal type of fluid motor will not function.

It is therefore the primary object of this invention to provide an improved reciprocating fluid motor assembly which includes a constant fluid supply, a cylinder and a piston, and fluid passages between the piston and the cylinder of such a nature whereby fluid under pressure is pumped only into the lower end of the cylinder and the fluid disposed in the lower end of the cylinder alternatingly urges the piston upwardly towards the upper end of the cylinder and permits the piston to slowly re-- turn to the lower end of the cylinder under the opposed.

pressure of fluid in the cylinder.

Another object of this invention is to provide an improved fluid actuated motor assembly of the reciprocating type, the fluid actuating motor assembly being of the type having a constant load thereon at all times with the load increasing and decreasing depending upon the particular stroke of the motor, and there being provided a pump for providing a constant flow of fluid under pressure and suitable valve means for controlling the direction of flow whereby the pressure in the cylinder in opposition to the movement of the piston due to the load imposed thereon is periodically increased and decreased so as to effect reciprocation of the piston.

A further object of this invention is to provide an improved actuated motor assembly of the reciprocating type, the motor assembly being driven by means of a" pump having a constant fluid output, and there being provided valve means for varying the effect of the fluid pumped by the pump on the motor so that the piston of the motor is constantly reciprocated within the cylinder thereof.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout, and in which:

FIGURE 1 is a diagrammatic view of the fluid actuated motor assembly which is the subject of this invention and shows the piston of the motor in its minimum load position at one end of the cylinder;

FiGURE 2 is a diagrammatic view similar to FIG- URE 2 and shows fluid being admitted to the cylinder under pressure so as to move the piston on a power stroke;

FIGURE 3 is a diagrammatic view of the fluid actumotor assembly and shows the piston at the end of its power stroke;

FIGURE 4 is a diagrammatic view showing the piston during its return stroke;

FIGURE 5 is a diagrammatic view of a modified form of fluid actuated motor assembly and shows the piston in an intermediate position during its power stroke;

FIGURE 6 is a diagrammatic view similar to FIG- URE 5 and shows the piston during its return stroke.

Referring now-tothe drawings in detail, it will be seen that there is illustrated in FIGURES 1 through 4, inclusive, a preferred form of the fluid actuated motor assembly which is the subject of this invention, the motor assembly being referred to in general by the reference numeral 10. The basic components of the motor assembly 10 include a fluid reservoir 12 for storing fluid at atmospheric pressure, a constant volume and pressure pump 14, a cylinder 16 and a piston 18 mounted in the cylinder 16 for reciprocatory movement.

The cylinder 16 is provided at one end with an end wall 20 and at the opposite end with an end wall 22. The end wall 22 carries a packing fitting 24 through which a piston .rod 26 extends. The piston rod 26 is connected to the piston 18 for movementtherewith. In order that this invention will operate, it is necessary that the piston rod 26 always has thereon a force urging the piston 18 towards the end wall 22.

Extending from the reservoir 12 to the pump 14 is a first fluid passage 28. The first fluid passage 28 has mounted therein a suitable filter 30 to assure the proper filtration of fluid passing from the reservoir 12 to the I pump 14. The filter 30 may be of any desired type.

Extending from the pump 14 and opening into the cyl inder 16 immediately adjacent the end wall 22 thereof is a second fluid passage 32. The fluid passage 32 serves to supply fluid from the pump 14 to the cylinder 16 to effect the movement of the piston 18 towards the end wall 20.

Opening into the cylinder 16 adjacent the end wall 20 is a third fluid passage 34. The opposite end of the third fluid passage 34 opens into the reservoir 12.

A fourth fluid passage is connected to the second fluid passage 32 adjacent the pump 14. This fluid passage is referred to in general by the reference numeral 36 and is connected to the fluid passage 34 intermediate the ends thereof. A fifth fluid passage 38 extends between the reservoir 12 and an intermediate portion of the second fluid passage 32.

Returning once again to the fluid passa es, it will be seen that the fluid passage 32 includes a first section 40 and a second section 42. The first section 40 is connected to the pump 14 whereas the second section 42 is connected to the cylinder 16. Disposed intermediate the sections 40 and 42 is a valve which is referred to in general by the reference numeral 44 and for the purpose of this invention will be considered a return valve. The fifth fluid passage 38 is also connected to the return valve 44 and is selectively communicated with the second fluid passage 32 thereby.

The third fluid passage 34 is also formed in sections and includes a section 46 and a section 48. The section 48 is connected to the fluid reservoir 12 whereas the section 46 is connected to the cylinder 16. The sections 46 and 48 are joined together at the intersection of the fluid passage 36 with the fluid passage 34.

The fourth fluid passage 36 is also formed in two sections including a first section 50 and a second section 52. The section 50 is connected to the section 40 intermediate the pump 14 and the return valve 44 whereas the section '52 is connected to the third fluid line 34. Disposed intermediate the sections 50 and 52 is a valve which is referred to in general by the reference numeral 54 and which is considered a reversing valve.

The reversing valve includes a valve housing 56 which defines a fluid passage 58 through which the passage of fluid is controlled by means of a valve assembly 60. The valve assembly 60 includes a partition wall 62 defining a valve seat for a ball valve 64. The ball valve 64 is urged into engagement with the valve seat by means of the spring 66. Also, the valve assembly 60 is of the check valve type and normally will not permit the flow of fluid through the fluid passage 58 into the remainder of the housing 56.

The housing 56 also defines a fluid passage 68 which terminates at the partition wall 62 and which at its opposite end opens into the fourth fluid passage 36. Opening into the fluid passage 68 is a cylinder 70 in which there is mounted a piston 72. The piston 72 is connected to the ball valve 64 by means of a piston rod 74. It is to be noted that the piston rod 74 extends through a portion of the fluid passage 58.

In order that the piston 72 may be periodically reciprocated at the desired time, the end of the cylinder 70 remote from the fluid passage 68 opens into a fluid passage 76. The fluid passage 76 has communicated therewith a control passage 78 whose opposite end is connected to the cylinder 16 by a short section 80. The section 80 is connected to the cylinder 16 intermediate the ends thereof and on the end wall 22 side of the piston 18 when the piston 18 is at the end of its power stroke. The section 80 has mounted therein a check valve assembly 82 which is of the type preventing the flow of fluid into the cylinder 16.

The control passage 78 also includes a short section 84 which is similar to the section 80, but is connected to the cylinder 16 adjacent the end wall 22, but on the end wall side of the piston 18 in its position at the end of its lower or return stroke. The section 84 of the control passage 78 is provided with a check valve assembly 86 which normally prevents the flow of fluid under pressure from the cylinder 16.

Referring to the check valves, it is also to be understood that there is provided a check valve assembly 88 in the section 46 of the third fluid passage 34, the check valve assembly 88 being of the type preventing the flow of fluid into the cylinder 16 through the third fluid passage 34.

The return valve 44 includes a valve housing 90 having a T-shaped fluid passage 92 therein which will communicate the fifth fluid passage 38 with the second fluid passage 32. Flow between the second fluid passage 32 and the fifth fluid passage 38 is controlled by means of a valve member 94. The valve member 94 is connected to a piston 96 which is mounted in a cylinder 98 formed in the housing. The valve member 94 engages a valve seat 100 to prevent flow of fluid from the valve 44 into the fifth fluid passage 38.

In order that movement of the valve member 94 may be accomplished, there is opened into the cylinder 98 remote from the valve member 94 a fluid passage 102. Communicated with the fluid passage 102 is a control passage 184 whose opposite end is connected to the section 50 of the fourth fluid passage 36. The piston 96 is provided with a bypass 106 therethrough so that normally a balance relationship exists and the valve member 94 is retained in a closed position.

The return valve 44 also includes a safety valve assemly which is referred to in general by the reference numeral 108. The safety valve assembly 108 includes a fluid passage 110 opening into the cylinder 98, but sealed therefrom by a lower extension 112 of the cylinder 96. The fluid passage 110 is separated from the fluid passage 102 by means of a ball valve member 114 which normally seats against the valve seat 116 and which is retained in this position by means of a coil spring 118. The coil spring 118 has the opposite end thereof bearing against an adjustable stop member 120 threadedly engaged in the valve housing 90. The valve member 114 is unseated only when there is an excessive buildup of pressure within the valve housing 90.

At this time it is pointed out that while the safety valve assembly 108 is highly desirable and in many instances required by state law, if no protection for machinery and other equipment is desired, the safety valve assembly 108 could be omitted.

Operation As was previously stated, there constantly remains on the piston rod 26 a load urging the piston 18 towards the end wall 22 of the cylinder 16. Assuming the piston 18 to be at the end of its lower or return stroke, as is illustrated in FIGURE 1, the valve members 64 and 94 will be in their closed positions. As a result, fluid will pass from the pump 14 through the second fluid passage 32 into the cylinder 16 intermediate the end Wall 22 and the piston 18. At the same time, the section 50 of the fluid passage 36 and the control passage 104 will be filled with fluid under pressure. As the fluid flows into the cylinder 16 the piston 18 is moved towards the end wall 20 in its power stroke.

The piston 18 continues to move towards the end wall 20 at a constant rate due to the constant flow of fluid from the pump 14. At such time as the piston 18 passes the connection of the section 80 into the cylinder 16, fluid begins to flow into the control passage 78 through the section 80, as is shown in FIGURE 3. The piston 18 continues in its travel towards the end wall 20 and as a result completely clears the section 80, as is shown in FIGURE 3.

When fluid is admitted to the control passage 78 under pressure, this fluid very quickly reacts on the piston 72 and urges it to the left. This results in the unseating of the valve member 64 and thus communicates the sections 50 and 52 of the fluid passage 36. As a result, the pressure of fluid in the fluid passage 104 drops and the pressures on opposite sides of piston 96 become unbalanced. As a result, the valve member 94 is moved to its open" position as is illustrated in FIGURE 3.

Inasmuch as the pump 14 is of a constant capacity and since the movement of the valve member 94 now communicates the fluid passage 32 with the reservoir 12 through the fluid passage 38, it will be seen that there is a bleeding off to the reservoir of a large portion of the fluid pumped by the pump 14 with the result that there now is insufficient pressure within the cylinder 16 to hold the piston 18 against the load imposed on the piston rod 26. As a result, the piston 18 will move towards the end wall 22 in its return stroke until such time as it reaches the end of its return stroke in FIGURE 1 at which time the section 84 is uncovered and the pressure in the control passage 78 is removed through fluid passage 34. At this time the valve member 64 returns to its closed position and the pressure is again built up in the control passage 104. This moves the piston 96 back to its original position to again seat the valve member 94.

When the present invention is used in conjunction with a reciprocating pump for oil wells, which is the primary intended use of the invention, as the piston 18 reaches its position illustrated in FIGURE 3, the load imposed upon the piston rod 26 will have been reduced to a minimum, this being the end of the pumping stroke. The load on the piston 18 at this time is equal to the weight of the sucker rod string and those components of the pump supported thereby, as well as the column of oil remaining in the tubing string. The reduction in load is minor in comparison to the over-all load.

Referring now to FIGURES and 6 in particular, it will be seen that there is illustrated a modified form of fluid actuated motor assembly, the motor assembly being referred to in general by the reference numeral 122. The motor assembly 122 is very similar to the motor assembly 10 with the exception of the fact that the return valve 44 has been eliminated.

The motor assembly 122 includes the reservoir 12, the pump 14, the cylinder 16 and the piston 18. The pump 14 is connected to the reservoir 12 in the same manner as in the motor assembly 10. Also, the reversing valve unit 54 remains the same. However, the fifth fluid passage 38, the return valve 44 and the control passage 104 have been eliminated. The second fluid passage 32 is now formed in a single section and the section 50 of the fourth fluid passage 36- has no longer connected thereto a control passage. The control passage 78 in its connections with the cylinder 16 remains the same.

Although there have only been shown two positions of the motor assembly 122, these positions corresponding to FIGURES 2 and 4, it is believed that the operation of the motor assembly 122 may be readily followed.

When the piston 18 is at its end of its return stroke, such as is illustrated in FIGURE 1, the valve 64 is seated and as a result the flow of fluid from the pump 14 is directly into the cylinder 16. The fluid entering into the cylinder 16 will move the piston 18 towards the end wall 26. This movement continues at a constant rate until such time as the piston 18 uncovers the connection 80 into the cylinder 16 at which time fluid under pressure enters into the control passage 78. The fluid under pressure entering into the control passage 78 reacts on the piston 72 and causes the unseating of the valve member 64. As a result, the sections 50 and 52 of the fourth fluid passage 36 are communicated and flow of a portion of the output of the pump 14 is through the fourth fluid passage 36 into the section 48 of the third fluid passage and back into the reservoir 12. Inasmuch as a portion of the output of the pump 14 is directed into the reservoir 12, and since the output of the pump 14 is constant, it will be seen that the pressure within the cylinder 16 will be reduced and as a result the piston 18 will be moved by the load thereon towards the end wall 22. The intermediate position of the piston 18 in its return stroke is illustrated in FIGURE 6. This return stroke continues until such time as the connection between the section 84 and the cylinder 16 is uncovered at which time the pressure within the control passage 78 is relieved and the valve member 64 is again permitted to seat. At this time the piston 18 reaches the end of its lower or return stroke and is now ready to start on the power stroke.

From the foregoing, it will be seen that there has been" devised two extremely simple forms of fluid actuated motor assemblies, which assemblies are of the selfoperating type and which utilize the output of constant vol-- ume and constant pressure pumps to operate a reciprocating fluid motor, which fluid motor is constantly under a load. In view of the extreme simplicity of the motor assembly the initial cost of the motor assemblies is relatively low and since there are very few moving parts, the maintenance of the motor assemblies is practically nil. For that reason, the motor assemblies are highly feasible for use in oil pumping units and the like, such as are normally found in oil fields. Other uses of the motor assemblies will become apparent.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention as claimed.

What is claimed as new is as follows:

1. A fluid actuated motor assembly comprising a cylinder, said cylinder having first and second ends, a piston mounted in said cylinder for movement between the first and second ends, a piston rod connected to said piston and extending out'of one end of said cylinder, a continuous load on said piston rod continuously uring said piston towards said second end, a removable load on said piston rod, said removable load being of the type whereby it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passage extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, and a control passage extending between the actuating means for said reversing valve and said cylinder and connected to said cylinder at spaced points ad jacent said first and second ends and intermediate the connections between said cylinder and said first and second fluid passages, fluid pressures in said control passage acting in a control capacity for said reversing valve, a first check valve in said control passage adjacent said first cylinder end restricting flow of fluid through said control passage at said first check valve to flow from said cylinder, a second check valve in said control passage adjacent said second cylinder end restricting flow of fluid through said control passage at said second check valve to flow into said cylinder.

2. A fluid actuated motor assembly comprising a cylinder, said cylinder having first and second ends, a piston mounted in said cylinder for movement between said first and second ends, a piston rod connected to said piston and extending out of one end of said cylinder, a continuous load on said piston rod continuously urging said piston towards said second end, a removable load on said piston rod, said removable load being of the type whereby .it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passages extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, and a control passage extending between the actuating means for said reversing valve and said cylinder and connected to said cylinder at spaced points adjacent said first and second ends and intermediate the connections between said cylinder and said first and second fluid passages, fluid pres sures in said control passage acting in a control capacity for said reversing valve, said reversing valve including a tor being controlled by fluid within said control passage, a first check valve in said control passage adjacent said firstcylinder end restricting flow of fluid through said control passage at said first check valve to flow from said cylinder, a second check valve in said control passage adjacent said second cylinder end restricting flow of fluid through said control passage at said second check valve to flow into said cylinder.

3. A fluid actuated motor assembly comprising a cylinder, said cylinder having first and second ends, a piston mounted in said cylinder for movement between said first and second ends, a piston rod connected to said piston and extending out of one end of said cylinder, a continuous load on said piston rod continuously urging said piston twards said second end, a removable load on said piston rod, said removable load being of the type whereby it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passage extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, and a control passage extending between the actuating means for said reversing valve and said cylinder and connected to said cylinder at spaced points adjacent said first and second ends and intermediate the connections between said cylinder and said first and second fluid passages, fluid pressures in said control passage acting in a control capacity for said reversing valve, said reversing valve including a valve unit in said third fluid passage normally preventing fluid flow from said first fluid passage to said second fluid passage, and an operator for said valve unit, said operator being controlled by fluid within said control passage, and a check valve in said second fluid passage intermediate said cylinder and the connection between said second and third fluid passage preventing flow of fluid from said third fluid passage into said cylinder.

4. A fluid actuated motor assembly comprising a cylinder, said cylinder having first and second ends, a piston mounted in said cylinder for movement between said first and second ends, a piston rod connected to said piston and extending out of one end of said cylinder, a continuous load on said piston rod continuously urging said piston towards said second end, a removable load on said piston rod, said removable load being of the type whereby it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passage extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, a fourth fluid passage open to exhaust, said fourth fluid passage being connected to said first fluid passage intermediate said cylinder and the connection between said first and third fluid passages, and a return valve controlling flow of fluid from said first fluid passage into said fourth fluid passage, a control passage extending from said third fluid passage intermediate said first fluid passage and the actuating means for said reversing valve to said return valve and fluid pressures in said control passages actuating of said reversing valve, a pressure relief by-pass through said return valve.

5. A fluid actuated motor assembly comprising a cylinder,.said cylinder having first and second ends, a piston mounted in said cylinder for movement between the first and second ends, a piston rod connected to said piston and extending out of one end of said cylinder, a continuous load on said piston rod continuously urging said piston toward said second end, a removable load on said piston rod, said removable load being of the type whereby it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passage extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, a control passage extending between the actuating means for said reversing valve and said cylinder and connected to said cylinder at spaced points adjacent said first and second ends and intermediate the connections between said cylinder and said first and second fluid passages, fluid pressures in said control passage acting in a control capacity for said reversing valve, a first check valve in said control passage adjacent said first cylinder end restricting flow of fluid through said control passage at said first check valve to flow from said cylinder, a second check valve in said control pas sage adjacent said second cylinder end restricting flow of fluid through said control passage at said first check valve to flow from said cylinder, a fourth fluid passage open to exhaust, said fourth fluid passage being connected to said first fluid passage intermediate said cylinder and the connection between said first and third fluid passages, and a return valve controlling flow of fluid from said first fluid passage into said fourth fluid passage.

6. A fluid actuated motor assembly comprising a cylinder, said cylinder having first and second ends, a piston mounted in said cylinder for movement between the first and second ends, a piston rod connected to said piston and extending out of one end of said cylinder, a continuous load on said piston rod continuously urging said piston towards said second end, a removable load on said piston rod, said removable load being of the type whereby it is imposed on said piston rod when said piston is adjacent said second end and removed from said piston rod when said piston is adjacent said first end, a pressurized fluid source, a first fluid passage extending from said pressurized fluid source to said cylinder adjacent said second end, a second fluid passage extending from said cylinder adjacent said first end to exhaust, a third fluid passage extending between said first fluid passage and said second fluid passage, a reversing valve disposed in said third fluid passage, said reversing valve including actuating means, and a control passage extending between the actuating means for said reversing valve and said cylinder and connected to said cylinder at spaced points adjacent said first and second ends and intermediate the connections between said cylinder and said first and second fluid passages, fluid pressures in said control passage acting in a control capacity for said reversing valve, a first check valve in said control passage adjacent said first cylinder end restricting flow of fluid through said control passage at said first check valve to flow from said cylinder, 21 second check valve in said control passage adjacent said second cylinder end restricting flow of fluid through said control passage at said second check valve to flow into said cylinder, and a check valve in said second fluid passage intermediate said cylinder and the connection between said second and third fluid passages preventing flow of fluid from said third fluid passage into said cylinder, a fourth fluid passage open to exhaust, said fourth fluid passage being connected to said first fluid passage intermediate said cylinder and the connection between said first and third fluid passages, and a return valve controlling flow of fluid from said first fluid passage into said fourth fluid passage, a second control passage extending from said third fluid passage intermediate said first fluid passage and said reversing References Cited in the file ofthis patent UNITED STATES PATENTS Helenberg July 21, 1936 Ringman June 23, 1942 Kyle et a1 June 27, 1943 10 Rose Aug. 1, 1950 Alward Sept. 4, 1951 Chenault Jan. 12, 1954 Zinty et a1. Dec. 13, 1955 Wildeman May 15, 1956 McAuley Sept. 23, 1958 Jones May 19, 1959 Johnston Nov. 24, 1959 Johnston Nov. 24, 1959 

